A flyback-type DC/DC converter is used for various power supply circuits such as an AC/DC converter. FIG. 1 is a block diagram illustrating a basic configuration of an AC/DC converter 100R reviewed by the present inventors. The AC/DC converter 100R mainly includes a filter 102, a rectifier circuit 104, a smoothing capacitor 106, and a DC/DC converter 201.
A commercial AC voltage VAC is input to the filter 102 via a fuse and an input capacitor (not shown). The filter 102 removes noise of the commercial AC voltage VAC. The rectifier circuit 104 is a diode bridge circuit that full-wave rectifies the commercial AC voltage VAC. An output voltage of the rectifier circuit 104 is converted into a DC voltage VIN, which is smoothed by the smoothing capacitor 106.
The isolated DC/DC converter 201 receives the DC voltage VIN at an input terminal P1, steps down the same, and supplies an output voltage VOUT, which is stabilized to a target value, to a load (not shown) connected to an output terminal P2. The DC/DC converter 201 includes a primary side controller 202, a photocoupler 204, a shunt regulator 206, an output circuit 210, and other circuit components. The output circuit 210 includes a transformer T1, a diode D1, an output capacitor C1, and a switching transistor M1. The topology of the output circuit 210 is that of a typical flyback converter.
As the switching transistor M1 connected to a primary winding W1 of the transformer T1 is switched, the input voltage VIN is stepped down and the output voltage VOUT is generated. Then, the primary side controller 202 stabilizes the output voltage VOUT to the target value by adjusting a switching duty ratio of the switching transistor M1.
The output voltage VOUT of the DC/DC converter 201 is divided by resistors R11 and R12. The shunt regulator 206 amplifies an error between the divided voltage (voltage detection signal) VS and a predetermined reference voltage VREF (not shown) which is internally set, and draws a forward current IF corresponding to the error from a light emitting element (light emitting diode) of an input side of the photocoupler 204 (sink).
A collector current IC corresponding to the forward current IF flows through a light receiving element (phototransistor) of an output side of the photocoupler 204. The collector current IC (feedback current IFB) is input to a feedback (FB) terminal of the primary side controller 202. A feedback voltage VFB having a negative correlation with the collector current IC is generated at the FB terminal. The primary side controller 202 switches the switching transistor M1 with a duty ratio corresponding to the feedback voltage VFB.
An output current IOUT of the DC/DC converter 201 varies depending on a state of the load. When the output current IOUT decreases while the switching transistor M1 is switched at a certain duty ratio, the output voltage VOUT rises. Then, since the forward current IF and the collector current IC increase and the feedback voltage VFB decreases, the duty ratio of the switching transistor M1 decreases and the current supply to the output capacitor C1 decreases to suppress the rise of the output voltage VOUT.
On the contrary, when the output current IOUT increases at a state that the duty ratio of the switching transistor M1 is constant, the output voltage VOUT decreases. Since the forward current IF and the collector current IC decrease and the feedback voltage VFB rises, the duty ratio of the switching transistor M1 increases and the current supply to the output capacitor C1 increases, so that the decrease of the output voltage VOUT is suppressed.
FIG. 2 illustrates a relationship between the output current IOUT and the collector current IC. Here, for simplicity of explanation, it is assumed that the conversion efficiency (gain) of the photocoupler 204 is 100% (IC/IF≈1).
As described above, the collector current IC decreases in a light load state where the output current IOUT is small, and increases in a heavy load state where the output current IOUT is large. An operating point of the circuit should be determined in consideration of the stability of the circuit. For example, as indicated by the solid line, if the collector current IC=0.5 mA is optimum at a rated current IOUT=IRATE, the collector current IC rises to about 1 mA in the light load state (IOUT≈0 mA). Assuming that IC=IF=1 mA and VOUT=24 V, the power consumption on the secondary side is 24 mW.
Due to the recent demand for energy savings, the reduction in power consumption of the light load or no-load state (also referred to as a standby state) is required, specifically a standby power of 100 mW or less is required in the entire AC/DC converter 100R. If the power consumption of 24 mW occurs on the secondary side of the DC/DC converter 201, it becomes difficult to suppress the AC/DC converter 100R as a whole to 100 mW or less.
It is assumed that the operating point of the circuit is determined such that the collector current IC in the light load state (IOUT≈0 mA) is, for example, 0.5 mA, as indicated by the alternate long and short dash line in FIG. 2. In this case, if the conversion efficiency of the photocoupler 204 is maintained at 100%, since the IC=IF=0.5 mA, the power consumption on the secondary side can be reduced to 12 mW but the proportion of the power consumption still exceeds 10% of the allowable power consumption of 100 mW.
When the output current IOUT increases up to the rated current IRATE according to the alternate long and short dash line, the collector current IC decreases up to, for example, 0.25 mA. Although the conversion efficiency of the photocoupler 204 has temperature dependency, the temperature dependency increases as the operating current decreases. Therefore, if the operating point is determined as indicated by the alternate long and short dash line, the gain of the feedback loop greatly fluctuates due to the temperature fluctuation, and the stability of the system degrades.
From the above, it is actually difficult to set the operating point such that the collector current IC decreases as indicated by the alternate long and short dash line.